The present disclosure relates to a flue gas desulfurization (FGD) system which is used to remove particulates, gases, and other contaminants from flue gas produced during combustion of medium- to high-sulfur fuels. In particular, sulfur dioxide (SO2), sulfur trioxide (SO3), HCl, and other acid gases can be captured; the acid dew point temperature of the flue gas can be reduced, and associated equipment corrosion can be lessened. Sorbents are used more effectively in the present system. This, among other things, increases boiler efficiency, enhances system corrosion resistance, improves material usage, reduces capital costs and operating costs, and improves capture of particulates and/or other contaminants.
During combustion in a boiler, the chemical energy in a fuel is converted to thermal heat, which can be used in various forms for different applications. The fuels used in the combustion process can include a wide range of solid, liquid, and gaseous substances, including coal (with low, medium, or high sulfur content), oil (diesel, No. 2, Bunker C or No. 6), natural gas, wood, tires, biomass, etc.
Combustion in the boiler transforms the fuel into a large number of chemical compounds. Water (H2O) and carbon dioxide (CO2) are the primary products of complete combustion. However, other combustion reactions with chemical components in the fuel result in undesirable byproducts. Depending on the fuel used, such byproducts may include particulates (e.g. fly ash), acid gases such as sulfur oxides (SOx) or halides (HCl, HF) or nitric oxides (NOx), metals such as mercury or arsenic, carbon monoxide (CO), and hydrocarbons (HC). The emissions levels of many of these byproducts will vary depending on the constituents found in the fuel, but can also be altered by the application of emissions control technologies.
The acid dew point temperature (ADP) is the temperature at which the acid gases in the flue gas are expected to begin condensing on the internal portions of the various system components in contact with the flue gas. Such acidic condensation results in corrosion of the system components, and is desirably avoided.
One means of avoiding this corrosion is by designing the heat recovery components so that the lowest expected temperature of the flue gas exceeds the ADP by a suitable margin. By doing so, however, some of the energy that is leaving the boiler envelope (as heat in the flue gas) is not captured. Unrecovered energy directly reduces the efficiency of the boiler, which has an unfavorable impact on the plant heat rate; the increased heat rate is equivalent to reduced plant efficiency. Reduced boiler efficiency also degrades the plant heat rate by requiring additional fan power to handle increased air and gas flows, as well as additional power in fuel and ash handling systems.
It would be desirable to provide systems and methods to remove particulates, gases, and other contaminants from the flue gas while also lessening equipment corrosion and/or improving the boiler efficiency and overall plant efficiency.